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Historical Events
Historical Events | Aug. 20, 2021

Pearl Harbor

Writing some 15 years after the Japanese attack, William F. Friedman said that the "Battle of Pearl Harbor is still being fought but the adversaries this time are all Americans."

This statement is almost as true today as it was in the 1950s.

On a par with theories about the Kennedy Assassination, the attack on Pearl Harbor continues to draw allegations of conspiracy, treason, and cover-up. The number of books written about the attack, each purporting to tell the "final truth," would fill a small library.

The following is not intended to be a comprehensive treatment of the Japanese attack on Hawaii. Instead, this article will try to draw together the earlier articles in this series about COMINT in the pre-war period, and tell the general story of December 7, 1941, the most momentous day for America in the Twentieth Century!


By the fall of 1941, the COMINT services in the U.S. military -- the Signal Intelligence Service for the Army and OP-20-G for the Navy -- both had small but vigorous programs with experienced personnel to exploit Japanese communications. The Army and Navy shared the processing of diplomatic messages -- decrypted from the Japanese PURPLE machine and codenamed MAGIC -- and were making efforts, so far mostly unsuccessful, against Japanese naval or military communications. American cryptologists, working with their British counterparts, had made some recoveries in a general purpose Japanese cryptosystem nicknamed JN-25, but, by late 1941, several basic changes to the system had foiled cryptanalysis and exploitation. The U.S. Navy was following the movements of major Japanese fleet elements by means of traffic analysis and Direction Finding from a series of sites around the Pacific.

NOW, ON WITH OUR STORY: The late 1930s were marked by increasing tensions between the United States and Japan. The principal source of conflict was the Japanese invasion of China and continued occupation of an immense area of the East Asian mainland. The United States was committed to the territorial integrity of China, and was supporting China's leader, Chiang Kai-shek.

The U.S. was the primary source for many Japanese imports, including high-quality scrap metal and aviation gasoline, two necessities for Japan's war in China. In 1940 the U.S. began requiring a license for export of these commodities, and then embargoed them, forbidding them to the Japanese until they withdrew their forces from China. The embargo brought U.S.-Japanese relations to the crisis point -- and it had the opposite effect it was intended to have.

Senior officials in the Japanese military and civilian ministries discussed the necessity of obtaining other sources of resources, since, due to the U.S. embargo, shortages of essential materials would soon be acute. Two main factions argued an aggressive policy: expand north into Siberia or southward into the East Indies and Indochina.

The Navy faction espousing southward movement won out, but strategists believed military action in the south would involve the Philippines and thus likely bring war with the United States. A preemptive strike against the U.S. fleet in Hawaii was a necessary prelude to any move in force into Southeast Asia.

After extensive planning, training, and refitting, a Japanese carrier strike force sailed to the Kurile Islands (then under Japanese control) and from there across the Pacific. The strike force left as early as November 15, 1941 and sailed under radio silence.

Meanwhile, Japanese diplomats in Washington were seeking to solve the international crisis by negotiations. The Japanese ambassador was Kurusu Saburo, a former navy officer, and to the Foreign Ministry, he seemed out of his depth in a time of crisis. To assist Ambassador Kurusu, Tokyo sent a professional diplomat, Nomura Kichisaburo.

Although they were not told of their Navy's plans to attack American bases in Hawaii, the instructions to the two ambassadors were to settle Japan's grievances with the United States by the end of November 1941, or something would happen automatically. [In the charged atmosphere after war broke out, many Americans believed the Japanese diplomats had been perfidious, pretending negotiations while Japan prepared its military strike. In actuality, the two ambassadors knew that negotiating time was short, but were not informed in advance about their military's plans for an attack on the U.S.]

The United States and Japan were far apart in their negotiating positions, however, and diplomacy was unable to narrow the gap. President Roosevelt proposed a personal meeting with the Japanese Emperor, but events overtook the proposal before it could be acted upon.

U.S. Navy COMINT personnel at intercept sites were following Japanese naval movements by traffic analysis. The Pacific DF net consisted of stations at Corregidor, Guam, Pearl Harbor, Dutch Harbor in the Aleutians, Samoa, and Midway Island. However, in November and December 1941, TA reports were sent to Washington by mail and were running two, sometimes three weeks behind.

The Japanese, realizing that Americans were monitoring their communications, had radio operators generating dummy traffic to mislead the eavesdroppers into thinking that some of the ships sailing through the North Pacific were still in home waters.

American intercept operators sometimes failed to get some of the traffic on Japanese naval vessels, leaving some ships unlocated. It should be noted, if there was no direct evidence that a Japanese ship was at sea, the American analyst would usually assume it was in its home port.

The Army and Navy COMINT organizations in Washington were cooperating on processing of Japanese diplomatic messages. Each service would handle about 25 messages a day for distribution to the nation's senior leadership.

Decrypts were disseminated by courier. OP-20-G's LCDR Alwin Kramer and the Army's COL Rufus Bratton would show the latest messages to a select group daily at 11 a.m. The select group began with the Director of Naval Communications and the Director of Naval Intelligence; they then gave messages of interest to the Naval Aide to the President, who showed them to Franklin Roosevelt. Colonel Bratton would take the messages to the Army recipients and the Secretary of State.

No copies of PURPLE decrypts were disseminated outside the continental United States. The PURPLE messages contained no military information anyway, they were strictly diplomatic traffic.

An important fact to remember is that both SIS and OP-20-G produced Communications Intelligence, but did not have the mission of interpreting it. Communications Intelligence was looked at in daily segments and for immediate, not long-term, benefit.

The international situation seemed dire enough from what could be read, and, on the basis of the diplomatic decrypts, general war warnings were sent to U.S. military commanders in Hawaii and the Philippines three times in November.

The commanders in Hawaii, General Short and Admiral Husband Kimmel took some action based on these warnings. General Short believed the real threat was less an attack from outside than the possibility of sabotage by Japanese living in the islands, and, among other actions, had Army Air Corps planes parked closely together on their runways so that could be more easily guarded.

At noon on Saturday, 6 December, Army SIGINT personnel solved and translated a message in which Tokyo instructed its ambassador, Nomura, to stand by for a 14-part message, a counterproposal to the Americans. He was instructed to present it to Secretary of State Hull at 1 p.m. EST on Sunday -- unbeknownst to Japan's diplomats in Washington, war was about to break out, but the emperor had directed that diplomatic niceties be observed and relations broken prior to the initiation of hostilities. The Japanese embassy was to destroy its cryptographic equipment once all fourteen parts had been received.

The destruction of cryptoequipment is a classic step prior to war, and this fact was not lost on the Americans.

The first thirteen parts of this important message were intercepted by the Navy station on Bainbridge Island. The intercept was teletyped to Washington, where it arrived around 3 p.m. Navy cryptanalysts tackled the message. The first thirteen parts proved to be an English-language text for presentation to the American government.

Bainbridge Island intercepted the 14th part about midnight (west coast time), and teletyped it on to Washington. The Army did the decryption of this part, which was in Japanese, and the services shared the distribution around the capital in accordance with usual practices.

With the 14-part message available, warnings were sent to American bases overseas. But, there were delays. Admiral Stark, Chief of Naval Operations, felt that previous warnings had been enough to keep Pearl Harbor alert, and declined to wake Admiral Kimmel in Hawaii at an early hour. General George Marshall, Army Chief of Staff, spent that Sunday morning in his usual recreational horseback ride and was unavailable until close to noon. He authorized dispatch of a war warning, but, as it happened, Army communications to Hawaii were down due to technical problems, and the warning was sent -- via Western Union telegram!

In the early morning hours of December 7th, Japanese aircraft from its carrier task force struck Hawaii -- not only the Navy base at Pearl Harbor, but also the Army installation at Schofield Barracks, the Army Air Corps's Hickam Field, and some smaller units. Their air operations continued for a little over two hours. The Japanese had achieved complete surprise.

American casualties were heavy in this, their first battle of the war. In addition to those wounded, the Navy lost 2,008 killed, the Marines lost 109, the Army lost 218, and there were 68 civilians killed. The Japanese lost 29 aircraft and crews.

In terms of war fighting capability, the Navy lost two battleships and two destroyers, with three battleships sunk but salvageable, and three additional battleships damaged.

Army Air Corps aircraft, closely parked on the runways by order of General Short, had been caught on the ground. The Army lost 96 airplanes, the Navy 92.

The one positive fact in the disaster was the fact that American aircraft carriers, the Japanese Navy's prime targets, had been delayed getting to Hawaii from a mission to deliver airplanes to other islands. They thus escaped destruction.

In the aftermath of the attack, General Short and Admiral Kimmel were relieved of their commands. Neither was tried by court-martial, but both received a good deal of official and public criticism. They went into retirement and spent the rest of their lives seeking official vindication.

The Japanese Ambassador received his Sunday, 1 p.m. appointment with Secretary of State Cordell Hull. Due to typing problems in their embassy, however, the diplomats were delayed for an hour. The attack in Hawaii had already occurred before they were ushered into Hull's office. According to one story, Hull cussed them out in plain down-home language and ejected them from his office.

The American public reacted in sorrow and anger to the news of the Japanese attack, and began demanding retaliation.

The day after, President Franklin Roosevelt addressed a joint session of Congress and requested a declaration of war against Japan. It was approved overwhelmingly.

Army cryptologists had been instructed to report in at mid-day on Sunday, December 7. When the news of the Japanese attack came, none were surprised at the fact of the attack, but all, including William Friedman, were surprised at the location.

John Hurt, one of the Army's Japanese translators, remembered that in the next few days he was called on quite a number of times to witness Last Wills written by SIS military personnel.

Even though there was no warning of the specific attack, the COMINT personnel in the Army and Navy had realized from diplomatic decrypts that war was very close. They also realized that victory was not inevitable. In a meeting on December 6, 1941, anxious civilians in SIS asked their military colleagues if the United States were strong enough to fight Japan and win. They received the answer, "I hope so."



  • William F. Friedman, Certain Aspects of 'MAGIC' in the Cryptological Background of the Various Official Investigations into the Pearl Harbor Attack, SRH-125, CCH Files.
  • John Hurt, In Retrospect, SRH-252, CCH Files.
  • Philip H. Jacobsen, "The Japanese Cipher Machines," NCVA (Paducah, Ky.: Turner Publishing Company, 1996), pp. 54-55.
  • Frederick D. Parker, Pearl Harbor Revisited: United States Communications Intelligence, 1924-1941 (Center for Cryptologic History, 1994).
  • L. F. Safford, The Undeclared War: History of R.I., 15 November 1943.
  • Anon. The Role of Radio Intelligence in the American-Japanese Naval War (August, 1941 - June, 1942), 1 September 1942, SRH-012, CCH files.